The present invention relates in general to combustible fuel slurries of solid carbonaceous material in liquid hydrocarbon fuel, and more particularly relates to the modification of such slurries to improve their utility and stability.
It is well known that in certain instances an atomizable liquid fuel offers a marked advantage over a solid fuel. However, the depletion of world liquid fuel reserves threatens the continued use of equipment adapted for such atomizable fuel. In addition, a comparative scarcity of certain liquid fuels in many lands coupled with an abundance of solid carbonaceous fuel has led to experimentation in extending the liquid fuels with solid fuels. Accordingly, attempts have been made to suspend the cheaper and more plentiful solid carbonaceous materials in liquid fuels in such fashion that the slurries could be used in place of the liquid fuels without extensive modification of the conventional equipment used to handle and burn the liquid fuels. Unless such slurries are treated in some fashion, however, the carbonaceous material will settle out of the slurry over time and form a hard, compact cake at the bottom of the slurry-containing vessel, thereby limiting the utility of the composite fuels.
Many approaches have been suggested to improve the stability and functionality of such composite fuels. One method is simply to grind carbonaceous material such as coal to practically colloidal size before adding it to the liquid fuel. The smallness of size inherently reduces sedimentation. However, this method is cost prohibitive because of the extensive milling required to reduce the particle size to that level.
It was also discovered that the composite fuel could be stabilized by the addition of coal distillates, such as tars and middle fractions, and subjecting the slurry to special heat treatment below the flash point of the mixture. This is inherently a very expensive batch process.
Other materials have also been added to the slurries in attempts to prevent settling of larger than colloidal size particles. Some processes have used fillers such as wood pulp, dust, and waste from starch, flour, and corn factories and others have used substantial amounts of colloidal size particles of coal to stabilize the non-colloidal particles.
Others have used emulsifiers which have generally required, in addition, substantial amounts of water which undesirably reduces the heating value of the fuel and has the potential of corroding the equipment used in handling and burning the slurry. Furthermore, emulsions are easily broken by heating or mechanical agitation and they are generally useful for stabilizing slurries containing only small amounts of coal.
Finally, still others have used additives to thicken and/or gel the composite fuel to prevent sedimentation of the larger coal particles. Casein, gelation, rubber, and soaps have been used to gel the hydrocarbon. Generally, however, these thickening agents have been employed in such quantity that they undesirably increase the viscosity and substantially decrease the number of uses for the composite fuel.
Accordingly, it would be desirable if a slurry of solid particulate carbonaceous material and liquid hydrocarbon fuel could be treated or modified so as to improve its stability and utility without requiring extensive grinding of the carbonaceous material, addition of excessive amounts of water, or complete thickening of the slurry.